1. Field of the Invention
The invention generally relates to techniques for reducing bandwidth usage and power consumption in a wireless voice communication system.
2. Background
Sub-band Coding (SBC) refers to an audio coder framework that was first proposed by F. de Bont et al. in “A High Quality Audio-Coding System at 128 kb/s”, 98th AES Convention, Feb. 25-28, 1995. SBC was proposed as a simple low-delay solution for a growing number of mobile audio applications. A low-complexity version of this coder was adopted by the early Bluetooth™ standardization body as the mandatory coder for the Advanced Audio Distribution Profile (A2DP). For the remainder of this application, this coder will be referred to as Low Complexity Sub-band Coder (LC-SBC). LC-SBC is a fairly simple transform-based coder that relies on 4 or 8 uniformly spaced sub-bands, with adaptive block pulse code modulation (PCM) quantization and an adaptive bit-allocation algorithm.
Recently, the Bluetooth™ standardization body adopted LC-SBC as the mandatory voice codec (coder/decoder) for wideband speech communication. However, since LC-SBC was originally intended for streaming audio, it does not embody some of the common and useful features that some other voice codecs use for mobile communication.
For example, it has been observed that only about 40% of a telephone conversation contains actual speech signals. The remaining 60% consists of regions of silence or background noise. Many voice coding algorithms try to take advantage of this fact by using either Discontinuous Transmission Modes (DTX) or Variable Rate encoding to reduce the average data rate. In the DTX mode, voice activity detection (VAD) logic identifies regions of the signal with no speech activity. In the absence of speech, the level of background noise is estimated and communicated to the decoder at a much lower rate that the speech regions. At the receiver side, Comfort Noise Generation (CNG) logic creates a signal approximating of the far end background noise. Variable Rate encoding attempts to achieve the same end goal by adapting the encoding mode (and bit-rate) as function of input signal characteristics. The coding mode is communicated to the receiver along with the compressed data.
Unfortunately, LC-SBC does not provide any of the foregoing features for reducing bandwidth usage and power consumption. What is needed, then, is an extension of LC-SBC that would make it more suitable for voice compression in the Bluetooth™ framework. The desired solution should provide reduced bandwidth usage and power consumption in a Bluetooth™ system used for wideband speech communication. Furthermore, the desired solution should not modify the underlying logic/structure of LC-SBC and have a relatively low impact on voice quality. Additional, the desired solution should be applicable to other sub-band codecs.